Effect of androstenedione on the growth and meiotic competence of bovine oocytes from early antral follicles

Lipopolysaccharide impairs the in vitro growth, steroidogenesis, and maturation of oocyte-cumulus-granulosa cell complexes derived from bovine early antral follicles

In postpartum dairy cows, lipopolysaccharide (LPS) derived from gram-negative bacteria causes uterine or mammary inflammation, resulting in low fertility. The present study aimed to investigate the effect of LPS on the in vitro growth (IVG), steroidogenesis, and maturation of oocyte-cumulus-granulosa cell complexes (OCGCs) derived from bovine early antral follicles. OCGCs were isolated from bovine early antral follicles (0.5-1 mm in diameter) and cultured in vitro for 12 days using media containing 0 (control), 0.01, or 1 μg/mL of LPS. The viability, cavity formation, and oocyte diameter of the OCGCs, as well as the concentrations of estradiol (E2) and progesterone (P4) in the IVG culture media, were determined. After IVG culture, oocytes collected from viable OCGCs were matured in vitro (IVM) in a medium without LPS. The nuclear maturation rate and the mitochondrial membrane potential of oocytes were determined. Bovine oocytes and cumulus-granulosa complexes derived from early antral follicles expressed genes encoding LPS receptor complex, such as toll-like receptor 4 (TLR4). Immunohistochemistry analysis further localized TLR4 expression predominantly in follicular granulosa and theca cells of early antral follicles. The viability of OCGCs and cavity formation in OCGCs were lower in the 0.01 and 1 μg/mL LPS groups than in the control group. No significant difference in oocyte diameter was observed between the treatment groups throughout the culture period. Moreover, E2 production was suppressed in the 0.01 and 1 μg/mL LPS groups from Days 4-8, whereas P4 production increased in the 1 μg/mL LPS group from Days 0-8. The nuclear maturation rate after IVM was lower in the 0.01 and 1 μg/mL LPS groups than in the control group. The mitochondrial membrane potential of post-IVM oocytes was lower in the 0.01 and 1 μg/mL LPS groups than in the control group. Taken together, these results indicate that LPS inhibited the growth and steroidogenesis of OCGCs and the meiosis and mitochondrial function of oocytes derived from early antral follicles. This study suggests that the detrimental effects of LPS on developing oocytes may contribute to long-term decreased fertility in postpartum dairy cows.

A refined culture system of oocytes from early antral follicles promotes oocyte maturation and embryo development in cattle

In brief

The proposed culture system improves the current state of in vitro culture of growing oocytes in the bovine species and allows access to the untapped gamete reserve, thus improving reproductive efficiency.

Abstract

The present study aimed to improve the in vitro culture of bovine oocytes collected from early antral follicles (EAFs) to support the progressive acquisition of meiotic and developmental competence. The rationale that drove the development of such a culture system was to maintain as much as possible the physiological conditions that support the oocyte growth and differentiation in vivo. To this extent, oocytes were cultured for 5 days, which parallels the transition from early to medium antral follicles (MAFs) in the bovine, and supports promoting a 3D-like structure were provided. Additionally, the main hormones (follicle-stimulating hormone, estradiol, progesterone, and testosterone) were added in concentrations similar to the ones previously observed in bovine EAFs. The meiotic arrest was imposed using cilostamide. The cultured cumulus-oocyte complexes (COCs) reached a mean diameter of 113.4 ± 0.75 µm and showed a progressive condensation of the chromatin enclosed in the germinal vesicle (GV), together with a gradual decrease in the global transcriptional activity, measured by 5-ethynyl uridine incorporation. The described morpho-functional changes were accompanied by an increased ability to mature and develop to the blastocyst stage in vitro, although not matching the rates obtained by MAF-retrieved oocytes. The described system improves the current state of in vitro culture of growing oocytes in the bovine species, and it can be used to increase the number of gametes usable for in vitro embryo production in animals of high genetic merit or with specific desirable traits.

L-Carnitine Supports the In Vitro Growth of Buffalo Oocytes

This study aimed to determine the effect of L-carnitine on the growth and subsequent nuclear maturation of buffalo small growing oocytes (92−108 µm in diameter) in vitro. Oocyte-granulosa cell complexes (OGCs) were dissected from early antral follicles of slaughtered buffaloes and cultured in in vitro growth (IVG) medium with the supplementation of different concentrations (0, 1.25, 1.875 or 2.5 mM) of L-carnitine for 6 days. The results revealed that L-carnitine increased the diameter of buffalo oocytes in vitro. The degeneration rate was significantly (p < 0.05) lower in 2.5 mM of L-carnitine-treated oocytes (10%) than others (55%, 45% and 32.5% in 0, 1.25 and 1.875 mM of L-carnitine-supplemented groups, respectively). The OGCs showed antrum-like structures significantly (p < 0.05) higher in the 2.5 mM of L-carnitine group (74.0%) than the 0- and 1.25-mM groups (34.6% and 38.1%, respectively). Furthermore, in vitro grown oocytes were placed in in vitro maturation (IVM) medium for 24 h to examine meiotic competence of in vitro grown oocytes with L-carnitine. The L-carnitine (1.875 and 2.5 mM) treated oocytes showed a higher rate of nuclear maturation up to the metaphase II (MII) stage and a lower rate of degeneration. In conclusion, L-carnitine enhances the growth, prevents degeneration, promotes the formation of antrum-like structures and supports nuclear maturation of buffalo oocytes in vitro.

17β-estradiol improves the developmental ability, inhibits reactive oxygen species levels and apoptosis of porcine oocytes by regulating autophagy events

OBJECTIVES

Estrogen plays a critical role in the development and apoptosis of oocytes. Autophagy is an evolutionarily conserved and exquisitely regulated self-eating cellular process with important biological functions including the regulation of reproduction. This study aimed to determine the effect of autophagy regulated by the biologically active form of estrogen (17β-estradiol) in porcine oocyte maturation in vitro.

MATERIALS AND METHODS

We measured the effects of oocyte developmental competencies and autophagic activity in the porcine oocyte regulated by 17β-estradiol using autophagic inhibitor (Autophinib). In addition, we studied the role of autophagy in reactive oxygen species (ROS) levels, mitochondrial distribution, Ca²⁺ production, mitochondrial membrane potential (ΔΨm), and early apoptosis by caspase-3, -8 activity in the mature oocytes.

RESULTS

The results showed that the oocyte meiotic progression and early embryonic development were gradually decreased with Autophinib treatment, which was improved by 17β-estradiol. Immunofluorescence experiments revealed that 17β-estradiol primarily could promote the autophagy in the mature oocytes, and block the reduced-autophagic events by Autophinib. Moreover, 17β-estradiol improved the Autophinib induced high ROS levels, abnormal mitochondrial distribution and low Ca²⁺ production in mature oocytes. Analyses of early apoptosis and ΔΨm showed that autophagy inhibition was accompanied by increased cellular apoptosis, and 17β-estradiol reduced apoptosis rates of mature oocytes. Importantly, autophagy was downregulated by treatment with Autophinib, an activation of caspase-8 and cleaved caspase-3 increased. Those effects were abolished by 17β-estradiol, which could upregulate autophagy.

CONCLUSIONS

Our study have showed important implications that 17β-estradiol could promote efficacy of the development of porcine oocytes, enhance the autophagy, reduce ROS levels and apoptosis activity in vitro maturation.

Biopsying, fragmentation and autotransplantation of fresh ovarian cortical tissue in infertile women with diminished ovarian reserve

STUDY QUESTION

Can ovarian biopsying per se and/or autotransplantation of fragmented ovarian cortical tissue activate dormant follicles and increase the number of recruitable follicles for IVF/ICSI in women with diminished ovarian reserve (DOR)?

SUMMARY ANSWER

Ovarian biopsying followed by immediate autotransplantation of fragmented cortical tissue failed to increase the number of recruitable follicles for IVF/ICSI 10 weeks after the procedure either at the graft site or in the biopsied ovary, but 12 of the 20 women subsequently had a clinical pregnancy during the 1-year follow-up.

WHAT IS KNOWN ALREADY

Infertile women with DOR constitute a group of patients with poor reproductive outcome mainly due to the low number of mature oocytes available for IVF/ICSI. Recent studies have shown that in vitro activation of residual dormant follicles by both chemical treatment and tissue fragmentation has resulted in return of menstrual cycles and pregnancies in a fraction of amenorrhoeic women with premature ovarian insufficiency.

STUDY DESIGN, SIZE, DURATION

This is a prospective clinical cohort study including 20 women with DOR treated at the fertility clinic, Rigshospitalet, Denmark, during April 2016-December 2017. Non-pregnant patients were on average followed for 280 days (range 118-408), while women who conceived were followed until delivery. Study follow-up of non-pregnant patients ended in September 2018.

PARTICIPANTS, MATERIALS, SETTING, METHODS

The study included infertile women aged 30-39 years with preserved menstrual cycles, indication for IVF/ICSI and repeated serum measurements of anti-Müllerian hormone (AMH) ≤ 5 pmol/L. Patients were randomized to have four biopsies taken from either the left or the right ovary by laparoscopy followed by fragmentation of the cortical tissue to an approximate size of 1 mm3 and autotransplanted to a peritoneal pocket. The other ovary served as a control. Patients were followed weekly for 10 weeks with recording of hormone profile, antral follicle count (AFC), ovarian volume and assessment for ectopic follicle growth. After 10 weeks, an IVF/ICSI-cycle with maximal ovarian stimulation was initiated.

MAIN RESULTS AND THE ROLE OF CHANCE

No difference in the number of mature follicles after ovarian stimulation 10 weeks after the procedure in the biopsied versus the control ovaries was observed (1.0 vs. 0.7 follicles, P = 0.35). In only three patients, growth of four follicles was detected at the graft site 24-268 days after the procedure. From one of these follicles, a metaphase II (MII) oocyte was retrieved and fertilized, but embryonic development failed. Overall AMH levels did not change significantly after the procedure (P = 0.2). The AFC increased by 0.14 (95% CI: 0.06;0.21) per week (P < 0.005), and the biopsied ovary had on average 0.6 (95% CI: 0.3;-0.88) follicles fewer than the control ovary (P = 0.01). Serum levels of androstenedione and testosterone increased significantly by 0.63 nmol/L (95% CI: 0.21;1.04) and 0.11 nmol/L (95% CI: 0.01;0.21) 1 week after the procedure, respectively, and testosterone increased consecutively over the 10 weeks by 0.0095 nmol/L (95% CI: 0.0002;0.0188) per week (P = 0.045). In 7 of the 20 patients, there was a serum AMH elevation 5 to 8 weeks after the procedure. In this group, mean AMH increased from 2.08 pmol/L (range 1.74-2.34) to 3.94 pmol/L (range 3.66-4.29) from Weeks 1-4 to Weeks 5-8. A clinical pregnancy was obtained in 12 of the 20 (60%) patients with and without medically assisted reproduction (MAR) treatments. We report a cumulated live birth rate per started IVF/ICSI cycle of 18.4%.

LIMITATIONS, REASON FOR CAUTION

Limitations of the study were the number of patients included and the lack of a non-operated control group. Moreover, 9 of the 20 women had no male partner at inclusion and were treated with donor sperm, but each of these women had an average of 6.8 (range 4-9) unsuccessful MAR treatments with donor sperm prior to inclusion.

WIDER IMPLICATIONS OF THE FINDINGS

Although 12 out of 20 patients became pregnant during the follow-up period, the current study does not indicate that biopsying, fragmenting and autotransplanting of ovarian cortical tissue increase the number of recruitable follicles for IVF/ICSI after 10 weeks. However, a proportion of the patients may have a follicular response in Weeks 5-8 after the procedure. It could therefore be relevant to perform a future study on the possible effects of biopsying per se that includes stimulation for IVF/ICSI earlier than week 10.

STUDY FUNDING/COMPETING INTEREST(S)

This study is part of the ReproUnion collaborative study, co-financed by the European Union, Interreg V ÖKS. The funders had no role in the study design, data collection and interpretation, or decision to submit the work for publication. None of the authors have a conflict of interest.

TRIAL REGISTRATION NUMBER

NCT02792569.

Effect of increased oxygen availability and astaxanthin supplementation on the growth, maturation and developmental competence of bovine oocytes derived from early antral follicles

In vitro growth (IVG) culture of bovine oocyte-cumulus-granulosa complexes (OCGCs) is generally carried out for 12 or 14 days using conventional gas impermeable culture devices. The culture duration may be longer compared to follicular development in vivo. During follicular development, follicles receive oxygen from micro vessels; however, oxygen supply is limited under the culture using conventional gas impermeable devices. The purpose of this study was to investigate the effect of increasing dissolved oxygen availability using a gas permeable (GP) culture device with or without antioxidant (astaxanthin, Ax) supplementation on 8-day IVG culture systems for bovine OCGCs derived from early antral follicles. We cultured OCGCs in GP, GP supplemented with Ax (GP + Ax), and a conventional gas impermeable device (control) for 8 or 12 days. OCGC viability were significantly higher when cultured for 8 days than 12 days (p < 0.001) in all culture condition, but significant difference was not observed between groups (p > 0.05). Antrum formation rates of OCGCs were higher after 12 days than 8 days of culture in all culture condition (p < 0.001) and were significantly higher in the control than GP groups regardless of Ax supplementation (p < 0.05). Oocyte diameters were similar among day-8 GP + Ax, day-8 control and day-12 control groups (p > 0.05). Nuclear maturation rates of oocytes grown in vitro for 8 days were significantly higher in the GP + Ax group than in the control and the GP groups (p < 0.05) and similar to oocytes grown for 12 days regardless of the culture conditions (p > 0.05). The generation of reactive oxygen species in OCGCs on day 8 of IVG culture was significantly lower in the GP + Ax group than those of the GP and control groups (p < 0.05). IVG oocytes after eight days of culture developed into blastocysts, and the cleavage and blastocyst rates were similar in all treatment groups. However, in vivo-grown oocytes had significantly higher (p < 0.05) cleavage and blastocyst rates than the IVG oocytes in all groups. The present study demonstrates that increased oxygen availability using a GP culture device with Ax supplementation promotes oocyte growth and maturation competence but inhibits proliferation of granulosa cells and antrum formation compared with a conventional gas impermeable culture device, and that OCGCs can attain developmental competence after 8 days of IVG culture.

Relationship between the antral follicle count in bovine ovaries from a local abattoir and steroidogenesis of granulosa cells cultured as oocyte-cumulus-granulosa complexes

The antral follicle count (AFC) is used as an indicator of cow fertility. We herein investigated the relationship between AFC and the steroidogenesis of granulosa cells and confirmed the developmental competence of oocytes derived from early antral follicles (0.5–1.0 mm) using in vitro growth culture. Slaughterhouse-derived ovaries were divided into high (≥ 25) and low (< 25) AFC groups based on AFC (≥ 2.0 mm). Oocyte-cumulus-granulosa complexes (OCGCs) collected from early antral follicles were cultured for 12 days. The total number, viability, and diameter of granulosa cells and estradiol-17β and progesterone production during the culture were evaluated. Surviving oocytes on day 12 were subjected to in vitro maturation, and their volume and nuclear status were evaluated. Some oocytes were subjected to the evaluation of developmental competence to blastocysts. Although the total number and viability of granulosa cells did not differ between the groups, granulosa cell diameters were smaller in the high AFC group than in the low AFC group. The estradiol-17β and progesterone ratio on day 8 was higher in the high AFC group than in the low AFC group. Oocyte volumes and nuclear maturation rates were greater in the high AFC group than in the low AFC group. The development rate to blastocysts was 9.1% in the high AFC group, while no oocytes developed to blastocysts in the low AFC group. Therefore, estradiol-17β production by granulosa cells appears to be greater in high AFC cattle than in low AFC cattle, thereby promoting the acquisition of oocyte competence.

Relationship between in vitro growth of bovine oocytes and steroidogenesis of granulosa cells cultured in medium supplemented with bone morphogenetic protein-4 and follicle stimulating hormone

Bone morphogenetic protein-4 (BMP-4) and FSH play important regulatory roles in follicular growth and steroidogenesis in vivo. The purpose of this study was to investigate the effects of BMP-4 and FSH on in vitro growth (IVG) and steroidogenesis of bovine oocyte-cumulus-granulosa complexes (OCGCs). We cultured OCGCs collected from early antral follicles (0.5-1 mm) in medium without BMP-4 and FSH for 4 days and investigated the appearance of OCGCs and their steroidogenesis. During the first 4 days of IVG, morphologically normal OCGCs produced more estradiol-17β (E₂), but less progesterone (P₄). Morphologically normal OCGCs were subjected to an additional culture in medium supplemented with BMP-4 (0, 10, and 50 ng/mL) and FSH (0 and 0.5 ng/mL) until day 12. We examined the viability and steroidogenesis of OCGCs after 8 and 12 days of culture. Oocyte growth, characteristics of granulosa cells, and the maturational competence of oocytes were also investigated. On day 8, the viability of OCGCs cultured without FSH was higher in the 10 ng/mL BMP-4 group than in the 50 ng/mL BMP-4 group (P < 0.05). No significant difference was observed in the viability of groups cultured with FSH, regardless of the addition of BMP-4, and FSH improved the viability of 50 ng/mL BMP-4 group similar to 10 ng/mL BMP-4 group. The total number of granulosa cells was larger in 10 ng/mL BMP-4 group cultured with FSH than in 50 ng/mL BMP-4 group cultured with FSH on day 8 (P < 0.05). E₂ production decreased from days 8-12, and P₄ production increased throughout IVG culture, regardless of the addition of BMP-4 and FSH (P < 0.05). No significant differences in E₂ production were observed between groups from days 4-8, regardless of whether BMP-4 was added without FSH; however, E₂ production in the group cultured with 50 ng/mL BMP-4 was suppressed by FSH. BMP-4 suppressed E₂ production from days 8-12, regardless of whether FSH was added. The group cultured with 10 ng/mL BMP-4 without FSH showed the lowest P₄ production among all groups for all culture periods. OCGCs that produced mature oocytes tended to secrete more E₂ and less P₄ than OCGCs that produced immature oocytes. In conclusion, until day 8 of the IVG culture, P₄ production by OCGCs was suppressed by the addition of 10 ng/mL BMP-4 in the absence of FSH, without inhibiting E₂ production. These conditions appear to mimic growing follicles until day 8 and mimic degenerating follicles from days 8-12 of culture.

Androgens promote the acquisition of maturation competence in bovine oocytes

Recent studies in mice suggest that androgens are important for normal follicle development. However, there have been few reports concerning the action of androgens in the growth of oocytes from large animals. The purpose of this study was to determine the roles of androgens in bovine oocyte growth in vitro. Oocyte-granulosa cell complexes (OGCs) collected from 0.4-0.7 mm early antral follicles were cultured for 14 days with 17β-estradiol (E2) and a non-aromatizable androgen, dihydrotestosterone (DHT). We also examined the ability of an androgen receptor (AR) inhibitor, hydroxyflutamide, to antagonize the effect of androgens on the oocytes. During growth culture, the OGC structures collapsed in the medium with DHT alone, while in the presence of E2, the OGC structures were maintained. In the medium with both androgens and E2, the mean diameter of oocytes was increased from 95 μm to around 120 μm, larger than those grown with E2 alone (115 μm). Also in the maturation culture, oocytes grown with androgens (A4 or DHT) and E2 showed higher percentages of metaphase II oocytes (63% or 69%, respectively) than those grown with E2 alone (32%). Moreover, these maturation rates were decreased by hydroxyflutamide in a dose-dependent manner. Immunostaining showed that ARs were expressed in oocytes and granulosa cells in early antral follicles, and the nuclei of granulosa cells showed intense AR expression. In conclusion, although E2 supports the OGC structure, additional androgens promote oocyte growth and their acquisition of meiotic competence via AR during in vitro growth culture.

Ovarian and adrenal androgens may be useful markers to predict oocyte competence and embryo development in older women

The aim of this retrospective analysis was to evaluate the association between serum levels of various hormones and in vitro fertilization (IVF) parameters in poor responder patients. Serum levels of anti-Müllerian hormone (AMH), dehydroepiandrosterone sulfate (DHEAS), androstenedione and testosterone were measured before the start of IVF treatment cycle. We found that serum AMH and DHEAS levels were positively correlated with the number of mature oocytes, fertilized oocytes and developed embryos, both in pregnant and in non-pregnant patients. In contrast, we found a positive correlation between serum androstenedione levels and IVF parameters in pregnant, but not in non-pregnant patients. In this latter group, androstenedione levels were positively associated with the number of mature oocytes and were negatively correlated with the number of fertilized oocytes. No correlation was observed with developed embryos. Finally, we reported a negative correlation between serum levels of testosterone and IVF parameters in pregnant women whereas no correlation was observed in non-pregnant patients. Our results suggest that serum AMH and DHEAS could be predictive for oocyte retrieval and embryo development. Moreover, the positive correlation between serum androstenedione levels and IVF parameters in pregnant patients, together with the lack of a consistent correlation in non-pregnant women seem to identify androstenedione as a more predictive marker for pregnancy outcome than testosterone. In addition, in our analysis testosterone shows a negative correlation with IVF parameters.

Steroid hormones promote bovine oocyte growth and connection with granulosa cells

Many approaches have been investigated for growing oocytes in vitro in mammals. To support oocyte growth in vitro, the culture systems must meet certain conditions for maintaining connections between oocytes and surrounding granulosa cells. The aims of this study were to determine the effects of combinations of 17β-estradiol (E2) and androstenedione (A4) on in vitro growth of bovine oocytes and to determine the number of connections between the oocyte and granulosa cells. Oocyte-granulosa cell complexes (OGCs) collected from early antral follicles (0.4-0.7 mm in diameter) were cultured for 14 days in a medium with different concentrations of E2 and A4, either alone or in combinations. We then assessed the number of transzonal projections (TZPs), which extend from granulosa cells through the zona pellucida to the oolemma. During in vitro growth culture, OGC structures were maintained in the medium with steroid hormones. The mean diameter of oocytes grown in the medium with both E2 and A4 was increased from 95.8 μm to around 120 μm, larger than oocytes grown without steroid hormones (109.9 μm) and similar in size to in vivo fully grown oocytes (119.4 μm) from 4- to 6-mm antral follicles. In subsequent in vitro maturation culture (22 hours), 30% (12 of 40) and 34% (14 of 41) of oocytes grown with E2 or A4 alone, respectively, matured to metaphase II; meanwhile, oocytes grown with a combination of E2 and A4 matured to metaphase II at a high rate (58%, 23 of 40). Growing oocytes isolated from early antral follicles had many uniformly distributed TZPs throughout the zona pellucida. After 14 days of culture, there was a significant decrease in the number of TZPs in oocytes grown without steroid hormones, whereas the number of TZPs was maintained in oocytes grown with steroid hormones. In particular, oocytes grown with E2 alone or with a combination of E2 and A4 had numbers of TZPs similar to oocytes before growth culture. In conclusion, a combination of E2 and A4 maintained the connections between oocytes and granulosa cells during in vitro growth culture of bovine oocytes for 14 days, resulting in the complete oocyte growth and the acquisition of meiotic competence in more than half the oocytes.

Production of fertile offspring from oocytes grown in vitro by nuclear transfer in cattle

Because of recent advancements in reproductive technology, oocytes have attained an increasingly enriched value as a unique cell population in the production of offspring. The growing oocytes in the ovary are an immediate potential source that serve this need; however, complete oocyte growth before use is crucial. Our research objective was to create in vitro-grown (IVG) oocytes that would have the ability to perform specialized activities, including nuclear reprogramming, as an alternative to in vivo-grown oocytes. Bovine oocyte-granulosa cell complexes with a mean oocyte diameter of approximately 100 μm were cultured on Millicell membrane inserts, with culture medium supplemented with 4% polyvinylpyrrolidone (molecular weight, 360,000), 20 ng/ml androstenedione, 2 mM hypoxanthine, and 5 ng/ml bone morphogenetic protein 7. Oocyte viability after the 14-day culture period was 95%, and there was a 71% increase in oocyte volume. Upon induction of oocyte maturation, 61% of the IVG oocytes extruded a polar body. Eighty-four percent of the reconstructed IVG oocytes that used cumulus cells as donor cells underwent cleavage, and half of them became blastocysts. DNA methylation analyses of the satellite I and II regions of the blastocysts revealed a similar highly methylated status in the cloned embryos derived from in vivo-grown and IVG oocytes. Finally, one of the nine embryos reconstructed from the IVG oocytes developed into a living calf following embryo transfer. Fertility of the offspring was confirmed. In conclusion, the potential of a proportion of the IVG oocytes was comparable to that of in vivo-grown oocytes.

Isolation of ovarian components essential for growth and development of mammalian oocytes in vitro

Mammalian ovaries contain a large number of oocytes, most of which degenerate either before or at various stages of growth. Dynamic and precise regulation in the ovary involves many factors, each with a unique role. Identifying the single most important factor is impossible; however, it may be possible to identify factors essential for oocyte growth. It is evident that oocytes can grow into competent ova in vitro; however, how faithfully the follicle should mimic the in vivo conditions remains unclear. In the culture system discussed in this review, bovine and mouse oocyte-granulosa cell complexes, at approximately the late mid-growth stage, spread on a substratum without the involvement of theca cells. The structural simplicity of this system is advantageous because it reduces the basic conditions essential for regulation of oocyte growth. Apart from biological factors, high concentrations of polyvinylpyrrolidone (molecular weight: 360000) improved oocyte growth. Among ovarian factors, androstenedione was used to compensate for the absence of theca cells, and it promoted both follicular growth and acquisition of oocyte meiotic competence. Most oocytes cultured in a group were viable after long-term culture, suggesting that unlike ovarian events, there was no exhaustive follicle selection. Collectively, oocytes and their associated granulosa cells can establish independent units capable of supporting oocyte growth in appropriately modified culture media.